1. Field of the Invention
The present invention relates to an immersion exposure method and immersion exposure apparatus.
2. Description of the Related Art
An immersion exposure method transfers a mask pattern image onto a substrate through an immersion medium while filling, with the immersion medium, at least one portion of the space between the substrate surface and the lower surface of a projection optical system. The immersion exposure method increases the resolution by taking advantage of the fact that the wavelength of exposure light in an immersion medium is 1/n times that in the air when the refractive index of the immersion medium is defined as n. The immersion exposure method can increase the resolution without greatly changing the arrangements of the conventional exposure apparatuses, and is therefore receiving a great deal of attention now (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2004-207710).
If the vapor pressure in an immersion exposure atmosphere is smaller than the saturated vapor pressure in immersion exposure, the immersion medium vaporizes from the surface in contact with the immersion exposure atmosphere. A variation in temperature of the immersion medium due to heat of vaporization varies the refractive index of the immersion medium. When the refractive index of the immersion medium varies, the imaging characteristics of the exposure apparatus optical system vary. This causes an error in position of an image formed on a substrate. When the substrate surface to be subjected to immersion exposure gets wet, the moisture in the substrate or the components and moisture of a photoresist arranged on the substrate migrate toward the substrate surface by the chromatography effect. The components that have migrated to the substrate surface mix with the immersion medium. This contaminates the immersion medium, resulting in a variation in its refractive index. As the substrate gets drier, the migrated substances alone remain on the substrate surface and disturb the annealing and development steps after exposure in pattern formation. Still worse, migration of the photoresist components warps the photoresist. To cope with these problems, a technique for preventing evaporation of an immersion medium to control a variation in its refractive index has been demanded.
As a means for solving the above problems, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-258662 proposes a method of specifying components of an immersion medium to prevent evaporation. However, this method requires adjustment of the components of the immersion medium in accordance with the substrate surface conditions and exposure environment. This complicates the adjustment operation.